The present invention relates to a method for supplying a slurry, particularly a slurry having an agglomerating property, and a slurry feeder suitable for use with a polishing apparatus for chemical mechanical polishing an object.
The present invention also relates to a slurry feeder for feeding a slurry (polishing fluid) to a main body of a polishing apparatus for polishing a surface of an object such as a semiconductor wafer to an even surface and a mirror-finished surface, a polishing apparatus having the main body and the slurry feeder, and a method for operation of the slurry feeder.
Hitherto, circuit wiring has become finer and distance between wires has become narrower as the integration of semiconductor devices develops. In particular, in a case of photolithography having a line width of at most 5 microns, an allowable focal depth is so shallow that a high degree of evenness on an imaging plane of a stepper is required. Therefore, a surface of a semiconductor wafer has to be made even. One of method for flattening a surface of a semiconductor wafer involves polishing the surface of the wafer with a chemical mechanical polishing apparatus.
FIG. 8 illustrates an example of an essential portion of a chemical mechanical polishing apparatus. This apparatus has a turntable 142 with a polishing cloth (a polishing tool) 140 attached on top thereof, a top ring 144 for rotatably pressing and holding a semiconductor wafer W as an object to be polished, and a slurry feed nozzle 146 for feeding slurry Q to the polishing cloth 140. The top ring 144 is connected to a top ring shaft 148 and held with an air cylinder (not shown) so as to be vertically movable. The top ring 144 has an elastic mat 150, e.g., a polyurethane mat, attached closely to bottom surface thereof, to hold the semiconductor wafer W. The top ring 144 also has a cylinder-shaped guide ring 152 disposed at an outer edge portion thereof in order to prevent the top ring 144 from dropping during polishing operations. The guide ring 152 is fixed to the top ring 144, and a bottom end surface of the guide ring 152 protrudes from a holding surface of the top ring 144 and is provided with a depressed portion inside a bottom end thereof for holding the semiconductor wafer W.
With this arrangement of the chemical mechanical polishing apparatus, the semiconductor wafer W is held under the elastic mat 150 below the top ring 144. The semiconductor wafer W is pressed against the polishing cloth 140 on the turntable 142 by the top ring 144, and is polished while rotating the turntable 142 and the top ring 144 and moving the semiconductor wafer W relatively to the polishing cloth 140. The slurry Q is supplied to the polishing cloth 140 from the slurry feed nozzle 146 during polishing operations.
In order to allow a good polishing of the semiconductor wafer W with the chemical mechanical polishing apparatus, a slurry feeder is required which can stably provide the chemical mechanical polishing apparatus with slurry (polishing or grinding fluid) at a constant concentration and a flow rate. The slurry feeder generally includes, for example, a stock solution tank for storing a stock solution of slurry, a preparation tank for adjusting concentration of slurry by diluting the stock solution with a deionized water (pure water), a chemical liquid or the like to a given concentration, a slurry supply tank for temporarily storing slurry adjusted in the preparation tank, and a slurry feed pipe for feeding the slurry from the slurry supply tank to the slurry feed nozzle 146 of the chemical mechanical polishing apparatus.
A conventional slurry feed pipe connecting the slurry supply tank to the chemical mechanical polishing apparatus adopts a so-called general circulation and supply system for discharging slurry via a roller pump from a circulating line to a table in the chemical mechanical polishing apparatus. This slurry feed pipe is provided with a circulating pipe for returning slurry discharged from the slurry supply tank back to the slurry supply tank, and a pipe branched from the circulating pipe for feeding the slurry to the chemical mechanical polishing apparatus. The slurry feed pipe is arranged so as to carry out the circulating operations for returning the slurry discharged from the slurry supply tank back to the slurry supply tank via circulating pump disposed in the circulating pipe, even if the chemical mechanical polishing apparatus is operated for polishing or is idling.
It is to be noted that slurry having an agglomerating property becomes more likely to agglomerate into particles having larger particle sizes, when the slurry is in a fluid state. Therefore, if such slurry is used for this invention, it may present a problem in that agglomeration of the slurry may be accelerated when the slurry is always in a fluid state due to circulating operations by virtue of the circulating pump in the manner as described above. In other words, the above feeding system cannot suspend circulation operations of the circulating pump. And thus the slurry has to be constantly circulated unless all chemical mechanical polishing apparatuses are brought into an idling status. Otherwise, agglomeration of the slurry would be accelerated.
In recent years, when manufacturing semiconductor devices, there is an increasing demand that plural device layers are formed on a semiconductor wafer. In order to accurately form plural device layers, it is necessary to make a surface of a layer covering each device layer flat and mirror-finished by utilizing a polishing apparatus. The polishing apparatus includes a main body having turntables, each rotating at an independent number of rotations, and also includes a top ring and a slurry feeder. Between a corresponding one of the turntables and the top ring is disposed an object such as a semiconductor wafer, and a surface of the object is polished to an even and mirror-finished surface by rotating this turntable while feeding a slurry for use during polishing.
The slurry feeder is required to supply a slurry (polishing fluid) continually to the polishing apparatus. In order to prevent interruption of supply of slurry during a process of polishing, a buffer tank is disposed which contains a slurry of a capacity that can polish at least one surface of a semiconductor wafer. The buffer tank is provided with a stirring device so as to stir the slurry well in order to prevent the slurry from remaining in the buffer tank and polishing particles from settling to make t concentration of the slurry irregular. The stirring device can stir the slurry in the buffer tank to keep a uniform concentration of the slurry to be fed to the polishing apparatus and enable polishing of the object at a high accuracy.
The present invention has been made with the above situation taken into account and an object of the present invention is to provide a method for supply of slurry, and a slurry feeder, which can provide a chemical mechanical polishing apparatus with slurry, including slurry with an agglomerating property, in an appropriate manner without causing the acceleration of agglomeration of the slurry.
Because such a conventional slurry feeder has a the buffer tank with a stirring device, however, an apparatus is rendered complex in structure and stirring causes a temperature of the slurry to rise so as to render a cooling load of the slurry high.
The present invention has been made with the above disadvantages taken into account and it has an object to provide a slurry feeder having a simplified structure capable of feeding a slurry having a uniform concentration, a polishing apparatus installed with the slurry feeder, and a method for operation of the slurry feeder.
In order to achieve the object as described above, the present invention provides a method for supply of slurry from a slurry supply tank, that stores slurry at a given concentration, to a chemical mechanical polishing apparatus for polishing an object, wherein operations of a slurry feed pump are suspended during a period of time other than during a time of the operations of feeding slurry to a chemical mechanical polishing apparatus.
The present invention also provides a method for feeding all slurry discharged from a slurry supply tank by virtue of a slurry feed pump to a chemical mechanical polishing apparatus while in operation.
The present invention is characterized in that a slurry feeder having a slurry supply tank for storing slurry at a given concentration, and a slurry feed pipe for feeding slurry from the slurry supply tank to a chemical mechanical polishing apparatus via a slurry feed pump, is provided with a control system for suspending operations of slurry feed pump for feeding slurry to the chemical mechanical polishing apparatus during a period of time other than during a time of feeding slurry to the chemical mechanical polishing apparatus in during a process of polishing.
The present invention is further characterized in that a plurality of turntables for use with a chemical mechanical polishing apparatus is disposed and that a supply pump is disposed for each of the turntables.
Moreover, the present invention is characterized in that a slurry feeder is provided with a preparation tank for adjusting a stock solution of slurry to a given concentration by mixing the stock solution thereof with deionized water or a chemical liquid, and for feeding the slurry of the given concentration to slurry supply tank. A control system is arranged so as to suspend circulating operations for returning slurry discharged from the preparation tank back to the preparation tank and so as to suspend stirring operations for stirring slurry in the preparation tank during a period of time other than during a time of adjusting concentration of slurry by diluting it in the preparation tank.
Additionally, the present invention is characterized in that a portion of a slurry supply tank connected to a slurry feed pipe is disposed so as to fail to discharge a slurry agglomerate, settled to a bottom portion of slurry supply tank, into slurry feed pipe by locating an exit for discharging slurry above the bottom of slurry supply tank.
In order to achieve the these objects, for example, as shown in FIG. 9, a slurry feeder according to the present invention comprises a slurry feeder 252 for feeding a given slurry to a polishing apparatus 251, and a slurry supply tank 212 for storing slurry to be fed to the polishing apparatus 251, wherein slurry is fed at a flow rate Q from slurry supply tank 212 to the polishing apparatus 251 and, when polishing particles in the g slurry is are allowed to settle at a sedimentation velocity V, a horizontal sectional area of slurry supply tank 212 is set to become smaller than Q/V. A configuration of feeding slurry at flow rate Q can be achieved typically by locating a pump for feeding slurry at the flow rate Q.
As the horizontal sectional area of slurry supply tank is formed so as to become smaller than Q/V, a vertical flow velocity of slurry in the slurry supply tank can be made greater than the sedimentation velocity of the polishing particles in slurry, and the slurry is allowed to be stirred well by flowing slurry in the storage tank and to sustain concentration of the slurry at a constant level. The slurry supply tank is configured such that, in usual cases, slurry enters from a vertical top portion thereof and it is discharged from a vertical bottom portion thereof. Sedimentation velocity of polishing particles in slurry means a velocity at which one polishing particle in slurry settles in a solution (typically deionized water) by virtue of gravity.
In order to achieve this object, the present invention provides a polishing apparatus, as shown in FIG. 9, which comprises a slurry feeder 252, a polishing table 242 slurry is provided from slurry feeder 252, and a slurry-returning line 308 through which slurry fed from slurry feeder 252 and not used for the polishing table 242 is returned to slurry supply tank 212.
With an arrangement of the polishing apparatus in the manner as described above, polishing can be effected by loading an object onto the polishing table and feeding slurry at a constant concentration from slurry feeder to the polishing apparatus, and slurry not used for the polishing table is returned to slurry supply tank for re-use by circulating this slurry. Because slurry in slurry supply tank is not stirred with a stirring device, a cooling load of slurry can be made small circulating the slurry. Further, concentration of slurry can be made constant when a returning flow velocity of slurry in a slurry-return line is set to be within a given scope in which concentration of slurry is made constant.
In order to achieve this object, the present invention provides a method for operation of a slurry feeder having a slurry supply tank for storing slurry to be fed to a polishing apparatus, wherein a flow rate of the given slurry to be fed from slurry supply tank to the polishing apparatus is set in such a manner that a flow velocity of slurry in slurry supply tank becomes faster than a sedimentation velocity of polishing particles in the slurry.
Because flow velocity of slurry in slurry supply tank is set to become faster than the sedimentation velocity of the polishing particles in the slurry, slurry can be fed to an object to be polished at a constant concentration.